AMDGPU: Defer creation of WWM VGPR spill slots

There's no reason to create these immediately. They can be created in
the prolog/epilog code like CSR spills. There's probably a cleaner way
to do this by utilizing the CSR spill code.

This makes the frame index used transient state for
PrologEpilogInserter, and thus makes serialization easier. Really this
doesn't need to be saved here but there isn't really a better place
for it.